Cardiac arrest is a significant public health problem cutting across age, race, and gender. Defibrillators have had a major impact on dealing with cardiac arrest in that they are the only reliable treatment for VF (ventricular fibrillation).
A positive impact on cardiac arrest survival has been demonstrated with the substantial reduction in time to defibrillation provided by the widespread deployment of automated external defibrillators (AEDs) and the use of implantable cardioverter defibrillators (ICDs). Examples of AEDs are described in U.S. Pat. Nos. 5,607,454, 5,700,281 and 6,577,102 while examples of ICDs are described in U.S. Pat. Nos. 4,407,288, 5,391,186, 5,405,363, and 7,383,085.
Research has been clear in demonstrating that the timing of resuscitation is of critical importance. For example, the probability of recovery goes down by about 5% per minute after the onset of ventricular fibrillation (VF) from a non-electrocution cardiac arrest. This knowledge led to the recent widespread deployment of AEDs, primarily in public areas with a high population concentration such as airports and shopping malls. A positive impact on cardiac arrest survival has been demonstrated due to the substantial reduction in time to defibrillation as a result of more available access to AEDs. In addition, for those patients identified as being at particularly high risk, an implantable cardioverter-defibrillator is often implanted in order to address episodes of cardiac arrest without the involvement of a rescuer.
One major challenge in the use of widely-deployed defibrillators is that defibrillation of a heart that has been in VF for a while can actually harm the heart. When the heart has been in VF for a long time, the delivery of the shock can actually lead to more dangerous rhythms such as asystole or EMD (Electro Mechanical Disassociation, a.k.a. Pulseless Electrical Activity or PEA). These problems occur after cardiac arrest because without continuing blood flow the oxygen and energy supplied to the heart tissue is no longer sufficient to enable it to contract with the necessary force to move blood in the case of PEA; and in the case of asystole can no longer even conduct an electrical signal. Shocking a heart in this condition is unlikely to result in a pulsatile rhythm.
In the case of VF, performing CPR-type chest compressions before defibrillation and minimizing the time to defibrillation shock following the cessation of the CPR chest compressions is important in facilitating effective recovery especially in cases of long duration VF. The primary purpose of administering cardio-pulmonary resuscitation (CPR) to a cardiac arrest victim is to cause blood to circulate into the heart before shocking it. This provides two benefits: first, the distended right ventricle is compressed back to its more nearly normal size, facilitating more effective defibrillation; second, the heart tissue has the potential to become more oxygenated in order to work effectively. Despite the importance of CPR-type chest compressions, they are often not performed in the field for a variety of reasons.
One approach that has been proposed for automating a treatment that can provide an effect similar to performing chest compressions is with the application of cardiac electrotherapy stimuli having an amplitude that is greater than that of pacing-type stimuli, but less than the amplitude and energy level associated with defibrillation-type stimuli. These are known in the art as medium voltage therapy (MVT). For example, U.S. Pat. No. 5,314,448 describes delivering low-energy pre-treatment pulses followed by high-energy defibrillation pulses, utilizing a common set of electrodes for both types of stimuli. According to one therapeutic mechanism of this pre-treatment, the MVT pulses cause chest constrictions similar to those of manual chest compressions of traditional CPR. The constrictions provide fresh oxygenated blood to the heart and facilitate a greater probability of successful defibrillation with a follow-on defibrillation pulse. U.S. Pat. No. 6,760,621 describes the use of MVT as pretreatment to defibrillation that is directed to reducing the likelihood of PEA and EMD conditions as a result of the defibrillation treatment. The mechanisms by which these results are achieved by MVT include a form of sympathetic stimulation of the heart. These approaches are directed to influencing the electrochemical dynamics or responsiveness of the heart tissues.
MVT has also been recognized as a way of forcing some amount of cardiac output by electrically stimulating the heart directly with stimuli that cause some heart and some skeletal muscle to contract in a controlled manner. See U.S. Pat. Nos. 5,735,876, 5,782,883 and 5,871,510. These patents describe implantable devices having combined defibrillation, and MVT capability for forcing cardiac output. U.S. Pat. No. 6,314,319 describes internal and external systems and associated methods of utilizing MVT to achieve a hemodynamic effect in the heart as part of an implantable cardioverter defibrillator (ICD) for purposes of achieving a smaller prophylactic device. The approach described in the '319 patent uses the MVT therapy to provide a smaller and less expensive implantable device that can maintain some cardiac output without necessarily providing defibrillation therapy.
Unlike a conventional AED or ICD, which operate with the primary purpose of restoring a normal cardiac rhythm, MVT stimulation can be used to provide cardiac output, which in turn causes perfusion of the heart and brain, as well as other critical body tissues. By providing perfusion of the heart and other vital organs, MVT prolongs the life of the patient even while the patient continues experiencing the arrhythmia. Additionally, MVT improves the likelihood of successful defibrillation or of a spontaneous return of circulation. An AED equipped with MVT can provide consistent high quality chest compressions. In the case of an implanted ICD, backup chest compressions provided by MVT can, in one sense, be even more important than in an AED, since in the case of the implantable device there may be no rescuer available to perform CPR when needed.
As heretofore envisioned, a combined defibrillation-MVT device utilizes separate defibrillation and MVT circuitry for generating and applying each type of electrotherapy since the magnitudes of these treatments can differ by an order of magnitude or more. Unfortunately, a number of challenges remain in practically incorporating MVT into defibrillation devices. Thus, improvements to a combined defibrillation-MVT device would be desirable.